Great advice there from Tom. The 6" scale has been my go to method for some time now. As a machinist with a home lathe I turned me a set of chamber gauges. I made them to minimum chamber dia according to SAAMI specs. Very quickly I found that most "Old" chambers were not cut to current SAAMI specs. 001" - 002" undersize makes absolutely no difference in the performance on a shotgun chamber, but it sure messes up the reading on a chamber gage.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra